The present invention relates to a coating comprising a photochromic material with an optically reversible photochromic effect, wherein the photochromic material has a switching threshold outside of or in the boundary region of the visible region of the spectrum, to a method of making a coating comprising a photochromic material with an optically reversible photochromic effect wherein the effect sets in at a certain wavelength, as well as to the use of such a coating.
According to the German textbook, "Rompp Chemisches Lexikon" (Chemical Encyclopedia), Volume 9, 1993 Georg Thieme Verlag, Stuttgart, N.Y., page 3403, a photochromic effect is described as the reversible property of a material, which is distinguished by a color change which takes place in the photochromic material when it is exposed to visible or ultraviolet light. However, some materials have a switching threshold for the onset of the photochromic effect which lies at the violet, high energy end of the visible region of the spectrum or at even shorter wavelengths, as a result of which the usability of such materials in the visible region of the spectrum is, at the very least, greatly limited.
It is an object of the present invention to increase the usability of photochromic materials in the optically visible spectral region of the electromagnetic spectrum.
This and other objects of the present invention are accomplished by a coating comprising a photochromic material coated with an influencing material such that the switching threshold of the coated photochromic material which forms a layer is shifted towards wavelengths longer than the switching threshold of the uncoated photochromic material.
The present invention further relates to a method wherein the photochromic material is acted upon by an influencing material.
By using an internal light effect, the switching threshold of the photochromic effect can be shifted to longer wavelengths and, thereby, to the optical region of the spectrum. As a result, less photon energy is required to utilize the photochromic effect. Since the sun has an intensity distribution within the visible region of the spectrum, in which the distribution maximum lies at a wavelength of about 500 nm, a large number of photons are available in daylight for the photochromic effect of the present inventive coating. This is expressed by a stronger absorption and, also a better coating color. Another advantage lies in the fact that, because of the shift in the switching threshold, a larger variety of lasers can now be used.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.